An internal combustion engine conventionally includes an engine block with at least one cylinder. Each cylinder accommodates a piston, which is connected to a crankshaft via a connecting rod and, in conjunction with a cylinder head, defines a combustion chamber. A mixture of air and fuel is introduced into the combustion chamber and ignited in cyclical manner, thereby producing rapidly expanding gases that drive linear movements of the piston, which in turn are converted into rotation of the crankshaft by the connecting rod.
The waste gases produced by the combustion of the fuel are emitted into the atmosphere via an exhaust system, which conventionally includes an exhaust manifold attached to the engine cylinder, an exhaust pipe extending away from the exhaust manifold, and one or more post-processing devices installed in the exhaust pipe in order to collect and/or alter the composition of the pollutants in the waste gases. Regarding these post-processing devices, internal combustion engines, and in particular diesel engines, may include a NOx trap (LNT).
The LNT is a catalytic device that includes catalytic agents such as rhodium. platinum and palladium as well as adsorbents, for example barium-based compounds, which have active positions configured to bind and trap the nitrogen oxides (NOx) contained in the exhaust gas.
When the quantity of NOx collected in the LNT exceeds a predetermined threshold value, the LNT undergoes a regeneration process or cycle, also referred to as DeNOx regeneration, the effect of which is to release and reduce the nitrogen oxides (NOx) that have accumulated in the LNT.
DeNOx regeneration is conventionally carried out by operating the internal combustion engine in a rich combustion mode. Rich combustion mode conditions are obtain when the air-fuel mixtures that are in the combustion chambers and have been ignited are in an air-fuel ratio that is lower than the stoichiometric value thereof (i.e. air-fuel equivalence ratio λ<1). During DeNOx, regeneration, these rich air-fuel mixtures are usually achieved by injecting one or more additional quantities of fuel into the combustion chamber after the main injection operation. These subsequent injections include small quantities of fuel that are injected into the combustion chamber after the piston in question has passed top dead center (TDC). In this way, the post-injection fuel is combusted in the combustion chamber without significantly increasing the torque applied to the crankshaft, but still raising the temperature and content of hydrocarbons (HC) and carbon monoxide (CO) in the exhaust gases. As they pass through the LNT, these exhaust gases deliver the energy needed to break the chemical bond at the barium position, so that the trapped NOx (particularly NO and NO2) are released. At the same time, the exhaust gases create an environment that is rich in HC/CO, so that the released NOx gases can be converted into molecular nitrogen (N2), carbon dioxide (CO2) and water (H2O) according to the following reactions:
                              NO          +          CO                →                                            1              2                        ⁢                          N              2                                +                      2            ⁢                          CO              2                                                          (        1        )                                                                    H              m                        ⁢                          C              m                                +                                    (                              n                +                                  m                  4                                            )                        ⁢                          O              2                                      →                                            n              ⁢              CO                        2                    +                                    m              2                        ⁢                          H              2                        ⁢            O                                              (        2        )            
In this context, some of the strictest environmental protection regulations demand that the performance capability of the LNT with regard to NOx conversion be tested with the engine running for diagnosing a possible malfunction and implementing counter-measures to prevent excessive NOx emissions. If the NOx emissions from the internal combustion engines are consistently below a predetermined threshold value, according to these regulations the performance capability of the LNT can be tested simply by determining the point in time when it completely loses the ability to convert even the smallest quantity of NOx (i.e. the LNT has failed completely).
One solution for determining the complete failure of the LNT includes using two NOx sensors, one upstream and one downstream of the LNT, to calculate the difference between the quantity of NOx the enters the LNT and the quantity that leaves the LNT. With this solution, it is possible not only to detect complete failure of the LNT, it also allows a reliable assessment to be made regarding its effectiveness at any point in its service life. However, the NOx sensors are relatively expensive, and they can consequently raise the overall costs of the exhaust gas system to unacceptable levels, particularly if the corresponding internal combustion engine is intended for small motor vehicles, city cars or other inexpensive vehicles.
In addition, other objects, desirable features and characteristics will become apparent from the subsequent summary and detailed description, and the appended claims, taken in conjunction with the accompanying drawings and this background.